1. Field of the Invention
The present invention generally relates to techniques for vibration testing of rolling element bearings in rotating machinery and, more particularly, is concerned with an improved method and apparatus for testing rolling element bearing condition.
2. Description of the Prior Art
The condition of rolling element (ball, roller, etc.) bearings used in rotating machinery is commonly assessed by measuring vibration acceleration signals generated by the rolling element bearings during machinery operation. While the signals which rolling element bearings produce contain energy throughout the spectrum, the majority of the energy is found in a band of frequencies typically ranging from a low frequency of 2,000 to 3,000 Hertz up to a high frequency ranging from 10,000 to 15,000 Hertz, using acceleration as a parameter.
Common measurement practice uses information derived from measuring the peak and average levels of signals in the time-domain and average levels derived from the frequency domain data of the same signals. Envelope detection, or de-modulation, of these signals for purposes of analysis is also a common practice.
Spectral analysis of the envelope detected signals is also commonly used as a method for distinguishing signal frequencies, calculable from bearing geometry, produced by rolling element bearing flaws, from interference signals although this method has several disadvantages. Spectral analysis returns only average information about the signals. Flawed rolling element bearings produce modified impulse type signals with high peak to average ratios. Spectrum analysis provides no information about peak signal levels, a good indicator of the potential for new or increased rolling element bearing damage and also for damage to the surrounding machine structures. In addition, dependence on spectral analysis of the raw signal or the envelope-detected signals before flawing occurs provides minimal information about causes of rolling element bearing damage including loss of clearance and inadequate lubrication.
Without interfering signals from other machine sources, analysis of the signals produced by the rolling element bearings is relatively simple. Since most machine interfering signals decrease more rapidly with increased frequency than the rolling element bearing signals, prior art practice typically minimizes, but does not eliminate, interference by measuring the signals at frequencies higher than the low frequency end of spectrum produced by the rolling element bearings. Because the rolling element bearing signals are attenuated rapidly with distance and more rapidly as measurement frequency is increased, reducing or eliminating the lower frequency portion of the spectrum used in the measurements adds significant error and uncertainty to the signal amplitude information.
Consequently, a need still exists for a method of testing rolling element bearing condition which will provide reliable and early detection of rolling element bearing flaws.